中嶋 誠二

BiFeO3 (BFO) thin films have been prepared on SrTiO3 (001) single crystal substrates by sputtering process which is suitable for mass production. Domain structure can be controlled by off-cut angle and direction of SrTiO3 substrates. A lateral PFM and a XRD reciprocal space mapping results reveal the BFO thin film on SrTiO3 (001) with 4 degrees off-cut along [110] is single domain. The single domain BFO shows well-saturated ferroelectric D-E hysteresis loops at R.T.

We report on fabrication of Pb(Zr,Ti)O-3 (PZT)/ZnO core-shell nanowires (NWs) on Pt/Ti/SiO2 membrane structure by metalorganic chemical vapor deposition (MOCVD). On the membrane structure, the diameter of ZnO NWs was smaller than those on Pt/Ti/SiO2/Si substrate. The growth temperature dependence of the diameter of ZnO NWs suggests that the difference in the diameter of ZnO NWs on the membrane and Pt/Ti/SiO2/Si substrate was caused by a lowering of the growth temperature on the membrane owing to insufficient heating. PZT was uniformly grown onto ZnO NWs on the membrane, and no distinct difference was observed in crystalline properties of PZT. These experimental results indicate that the crystalline properties of PZT/ZnO core-shell NWs on the membrane is as good as those on the surrounding area, and also open the door to novel NW-based piezoelectric vibration energy harvesters in combination with a mechanical resonance of "soft" substract.

BiFeO3 (BFO) thin films have been deposited on SrRuO 3/SrTiO3 (001) substrate by using ion beam sputtering process. At low oxygen partial pressure of 11 m Pa, rhombohedral and large c/a mixed phase thin film have been obtained in spite of rhombohedral BFO single phase formation at high oxygen partial pressure of 73 mPa. From wide area 2θ-ψ mappings, diffraction peaks from large c/a phase BFO thin film were obtained with the same extinction rule as those of rhombohedral BFO. Reciprocal space mappings around BFO (003) and BFO (103) spots indicate that lattice parameters of large c/a phase BFO were a = 0.381 nm and c = 0.461 nm (c/a = 1.22), respectively. Moreover ferroelectric domain switching could be observed in both of rhombohedral BFO and mixed phase BFO thin films. © 2011 Materials Research Society.

PbTiO3-covered ZnO nanorods were grown on Al2O 3(1120) by metalorganic chemical vapor deposition (MOCVD), and their crystalline orientation was investigated by x-ray diffraction (XRD). Structural analysis by scanning electron microscopy and XRD revealed that the hexagonal ZnO nanorods had {1010}-side facets. XRD analysis of PbTiO3 thin films on ZnO(1010)/Al2O3(1010) revealed that PbTiO3 was epitaxially grown on ZnO(1010), showing 6 variants of crystallites with the c-axis tilted either 27° or 69° from the surface normal to the ZnO(1010) plane. Effective piezoelectric coefficients calculated for the 27° and 69°-crystallites using piezoresponse force microscopy confirm that deformation of nanorods and nanotubes contributed to the large electrically-induced strain along the radial direction. © 2011 Materials Research Society.

350-nm-thick BiFeO3 (BFO) thin films have been deposited by dual ion beam sputtering on SrRuO3-buffered SrTiO3 (001) substrates of 0 degrees and 0.5 degrees off-cut along [100] at various Bi2O3-side/Fe2O3-side beam current ratio. The BFO thin film on 0.5 degrees off-cut STO substrate deposited at beam current ratio of 20 mA/30 mA show fully saturated D-E hysteresis loops at room temperature. The remanent polarization (2P(r)) was 132 mu C/cm(2). Stripe domain structure was also observed in IP piezoelectric response image. It is considered that the improvement of leakage of BFO on 0.5 degrees off-cut STO substrate is due to reduction of 109 degrees domain walls.

We have investigated relationships between leakage current and microstructure or domain structure of BiFeO3(BFO) thin films, and leakage current mappings of BFO thin films have been performed by current sensitive AFM. 350-nm-thick and 250-nm-thick BFO thin films were prepared on Pt/TiO2/SiO2/Si substrate by pulsed laser deposition (PLD) and chemical solution deposition (CSD), respectively. Average grain size of PLD-BFO thin film is about 480 nm, which is the same as the film thickness. From the leakage current mapping at a bias voltage of-16 Vdc, leakage current of the BFO thin film flows through not only grain boundary but also the grain itself. On the other hand, CSD-BFO thin film shows rosette structure and small size grains. From the leakage current mapping at a bias voltage of -10 Vdc, leakage current flows along boundaries of the rosette structures. These results indicate that leakage current of BFO strongly depends on its microstructure. © 2010 Materials Research Society.

MOCVD-Pb(Zr,Ti)O_3(PZT)薄膜のグレインサイズを, Ir下部電極のグレインサイズおよびPZT薄膜の成長速度を変化させることで制御することができた。Ir/PZT/Irキャパシタにおいては, Ir下部電極のグレインサイズが50nmから200nmまで増加するのに伴い, PZT薄膜のグレインサイズも120nmから240nmまで増加した。グレインサイズの増加とともに比誘電率, 残留分極は増加し, 抗電界は減少した。このような電気的特性のグレインサイズ依存性はバルクセラミックスにおけるサイズ効果と同じであった。

An Influence of stress of crystal structure of polycrystalline BiFeO 3 (BFO) thin film on membrane structure has been investigated. To confirm the stress dependence of the crystal structure, reciprocal space mapping measurement of polycrystalline BFO thin films on Pt (200 nm)/TiO2 (50 nm)/SiO2 (600 nm)/Si (625 μm) plate substrate and Pt (200 nm)/TiO2 (50 nm/SiO2 (600 nm)/Si (15 μm) membrane substrate have been performed. These BFO thin films have been prepared by pulsed laser deposition (PLD). The obtained BFO thin films were polycrystalline and mainly oriented to (001) and (110) plane. From reciprocal space mapping measurement, (110) oriented BFO grains on Pt/TiO2/SiO2/Si (15 μm) membrane substrate were expanded perpendicularly to the film plane about 0.15% and compressed in parallel to the film plane about 0.7% comparing to that on Pt/TiO2/SiO2/Si (625 μm) plate substrate. And (001) oriented BFO grains on the Pt/TiO2/SiO2/Si membrane substrate were expanded about 0.20% perpendicularly to the film plane and compressed about 1.3% in parallel to the film plane comparing to that on Pt/TiO2/SiO2/Si (625 μm) plate substrate. © 2008 Materials Research Society.

BiFeO3 (BFO) thin films have been deposited on La (3.73 wt%) doped-SrTiO3 (La-STO) (001), (110) and (111) single crystal substrates by pulsed laser deposition (PLD). X-ray diffraction (XRD) measurements have been performed for confirming structural properties, and electric properties have also been measured at 80 K. From XRD θ-2θ measurements, perovskite single phase BFO thin films are epitaxially grown on La-STO (001), (110) and (111) substrates. D-E hysteresis loops measurements at 80 K give remanent polarization (Pr) of 62, 110 and 147 μC/cm 2, respectively. These agree well with remanent polarizations measured by pulsed polarization method applying positive-up-negative-down (PUND) pulses. © 2009 Materials Research Society.

Bi-layer-structured mutiferroic Bi5Ti3FeO 15 (BTFO15) (m = 4) and natural-superlattice-structured Bi 4Ti3O12- Bi5Ti3FeO 15 (BIT-BTFO15) (m = 3-4) thin films have been prepared on (001) and (110) oriented SrTiO3 (STO) single crystal substrates by using pulsed laser deposition. X-ray diffraction patterns of these thin films on (001) STO single crystals shows the obtained thin films were (00l)-oriented layer-perovskite single phase, and BIT-BTFO15 (m = 3-4) natural-superlattice- structure has also been obtained. On (110) STO single crystal, layer perovskite (11l) oriented thin films have also been obtained. For characterizing ferroelectric properties, these thin films have been prepared on (001) and (110) oriented La-doped (3.73 wt%) STO single crystal substrates. From ferroelectric D-E hysteresis loops measurements, BTFO15 (m = 4) and BITBTFO (m = 3-4) thin films on (110) La-doped STO single crystals shows good ferroelectric hysteresis loops and their double remanent polarizations (2Pr) were 47 μC/cm2 and 44 μC/cm2, respectively. However, these thin films on (001) La-doped STO single crystals do not show ferroelectric characteristics. © 2008 Materials Research Society.

We have investigated the effect of Bi-excess surface layers of stoichiometric BiFeO3 thin films prepared by chemical solution deposition. A stoichiometric BiFeO3 thin film with both the Bi-excess top and bottom surface layers shows improved crystallinity with the remanent polarization of 65 μC/cm2 at 80 K, which is larger than BiFeO 3 film prepared by the same process using stoichiometric solution. These results are attributed to the reduction of the imperfect crystal at the interface between BiFeO3 film and electrode. By inserting Bi-excess layers, the saturated magnetization of all the films becomes smaller than that of the film using stoichiometric solution. Preparing stoichiometric BiFeO 3 thin films with Bi-excess surface layers is an effective way to obtain good ferroelectric property. © 2008 Materials Research Society.

Multiferroic BiFeO3 thin films have been prepared on Pt/TiO 2/SiO2/thick (200 μm) and membrane (15 μm) Si substrates by pulsed laser deposition (PLD) to confirm the influence of stress from substrate. The Si membrane was obtained by etching using reactive ion etching (RE) until thickness is 15 μm. The X-ray diffraction peaks of BiFeO3 thin film on Pt/TiO2/SiO2/Si (15 μm) membrane substrate slightly shift to lower angles, compared to those on Pt/TiO2/SiO2/Si (200 μm) substrate. Ferroelectric hysteresis loops were also measured at 150 K before and after Si etching by RE. The BiFeO3 thin film on the Pt/TiO2/SiO2/Si (15 μm) membrane structure shows remanent polarization (Pr) of 95 μC/cm2 for a maximum applied voltage of 18 V, which is larger than Pr = 71 μC/cm2 of BiFeO3 thin film on Pt/TiO2/SiO2/Si (200 μm) substrate at the same measurement conditions. Under magnetic field of 1.1 T, remanent polarization (Pr) of BiFeO3 thin film on Pt/TiO2/SiO 2/Si (15 μm) membrane structure increased from 95 μC/cm 2 to 101 μC/cm2 at 150 K. © 2007 Materials Research Society.

The grain size of MOCVD-Pb(Zr,Ti)O-3(PZT) thin films was successfully controlled by changing the grain size of Ir bottom electrodes and by changing the growth rate of PZT films. In Zr/PZT/Ir/SiO2/Si capacitors, the grain size of PZT thin films increased from 120 to 240nm as the grain size of bottom Ir electrodes increased from 50 to 200nm. The dielectric constants of PZT thin films increased from 760 to 1440 as the grain size increased from 120 to 240nm. Remanent polarization increased and coercive field decreased as the grain size increased. This dependence of electrical properties on the grain size coincided with that of ceramics.

The effect of bottom electrode thickness on the electrical properties of PZT capacitors with Ir and IrO2 electrodes was investigated, with particular attention to switching endurance characteristics. Ir and IrO2 electrodes were prepared by rf magnetron sputtering. PZT films were grown by MOCVD. Secondary ion mass spectrometry (SIMS) analysis showed thick Ir and IrO2 electrodes performed well as a barrier to the PZT elements. On the other hand, strong diffusion at the interface between PZT and the electrodes was observed, when the Ir and IrO, electrodes were thin. From transmission electron microscope (TEM) observation, it was also found that there was an amorphous intermediate layer at the interface between the PZT and the thick Ir bottom electrode. The switching endurance characteristics were influenced by the thickness of the Ir bottom electrode.